Method of reducing crosstalk in processing of acoustic or optical signals

ABSTRACT

A method for reducing frequency crosstalk in the transmission of digitized audio signals. Signals from partial bands in which spectral components of specific frequencies occur as signal components and signals from partial bands in which spectral components occur as crosstalk components in the stop range, undergo a weighted summation. Following transmission, the partial band signals undergo an inverse operation to weighted summation. The method operates independently of the selected encoding process, and is consequently universally usable.

BACKGROUND OF THE INVENTION

The invention relates to a method for reducing frequency crosstalkduring the transmission and/or storage of acoustic signals.

The more it proves possible to reduce the data quantity to betransmitted without any noticeable quality loss, the more attractive isthe transmission and/or storage of digitized acoustic signals. Numerousmethods are known for data compression of acoustic signals bytransformation of the digitized data in the frequency range or, asubdivision into different frequency bands.

The subdivision into several partial bands can take place insingle-stage manner by a filter bank or in multistage manner by theseries-connection of two or more filter banks. (A following filter bankcan also be replaced by a transformation.) The thus prepared data arecompressed to reduce the data quantity utilizing the signal statisticsand the psychoacoustics in such a way that, following transmission andinverse transformation of the data, as far as possible the human earperceives no difference compared with the input signal. In the proposalfor the standardization of data compression methods for data audiosignals of the International Organization for Standardization, a hybridfilter bank is used to subdivide the signal spectrum into partial bands.The analysis bank in the coder consists of two stages. Firstly thespectrum of the input signal is subdivided into 32 partial bands by apolyphase filter bank, such as is described, for example, by H. J.Nussbaumer, M. Vetterli, "Computationally Efficient QMF Filter Banks",IEEE Proc. ICASSP 1984, p. 11.31-4. Each of these partial bands is thensubdivided once again into 12 bands with a following TDAC filter bank.Such a TDAC filter bank is described by J. P. Princen, A. W. Johnson andA. B. Brodley, "Subband/Transform Coding Using Filter Bank Designs Basedon Time Domain Aliasing Cancellation", Proc. ICASSP '87, p. 50.1.1-4.

In the publication by S. A. Towns, T. K. Trong, the VSLI design of asubband coder, International Conference on Acoustic Speech and SignalProcessing, vol. 2, 1984, New York, 34B 21-24B 24 a comparison takesplace of a filter bank having a tree structure and a filter bank havinga parallel structure with respect to the design of subband coders inVSLI technology.

A prerequisite for the use of serial separator stages in datacompression methods is a good band separation, so that as far aspossible each spectral component of the input signal influences only onepartial band signal and therefore quantization errors, which occur inthe partial bands, influence only the associated frequency range in theoutput signal.

Therefore the partial band filters of the overall system must have avery high stop band attenuation. The serial arrangement of filter and/ortransformation stages gives rise to an inadequate stop band attenuation,however, because in each case several partial bands occur in thetransition ranges of preceding separator stages. In the frequencyresponse of the corresponding partial band filter, this leads to a clearrise of the signal outside the pass band.

Therefore the spectral components of the input signal in thecorresponding frequency ranges, after passing through the separatorstages, influence the partial band signals in the form of crosstalkcomponents. Following the combination of the partial band signals,quantization errors in one of these partial bands correspondinglyinfluence frequency ranges outside the particular partial bands.

In known methods an attempt is made to minimize deterioration of theoutput signal due to frequency crosstalk by taking account of thiseffect during coding. However, this is only possible to a limited extentand makes the coding processes more complex.

SUMMARY OF THE INVENTION

The object of the invention is to provide a method for reducingfrequency crosstalk, which is particularly effective and operatesindependently of a following coding.

This object is achieved according to the invention, by performing aweighted summation of the signals from the partial bands (in whichcorresponding signal or crosstalk components occur) followingsubdivision. The resulting weighted partial band signals aresubsequently coded and transmitted and/or stored and decoded. In orderto eliminate the signal change caused by the weighted summation, thedecoded partial band signals undergo a process which is inverse to theweighted summation, prior to combination.

For a given system for subdividing the signals into frequency bands, inwhich the subdivision takes place in series-connected stages, theposition and phase relationship of the signal components of thecorresponding crosstalk components can be determined.

In another embodiment of the invention, which offers advantages if useis made of a preceding filter stage, the input signal is subdivided intoan even number M of partial bands, whereby the spectral components arereflected in the partial bands. In this case the output signals of everyother partial band of this filter stage are corrected before the signalis supplied to the following stage. Such correction may be performed bymultiplying the partial band signals with the following form {1,-1,1-1 .. . }, which cancels out the reflection of the partial band spectra.

In still another embodiment, the weight factors for the summation areoptimized for the frequency response of the series-connected stages. Ina following filter stage with a subdivision of a signal into an evennumber of N partial bands, N/2 values of c_(m) are determined.

The total frequency response of the transmission using the methodaccording to the invention is significantly improved compared with knownmethods. Moreover, the method according to the invention gives rise tono additional signal falsifications; that is, if the partial bandsignals are combined without encoding and decoding, the input/outputbehavior of the overall system is not affected by the method, because asa result of the inverse operation the original partial band signals arereconstructed in error-free manner.

The method according to the invention can be used in universal manner.As the reduction of the frequency crosstalk is independent of the codingprocesses used, it is suitable for use in the transmission of bothacoustic and optical signals.

On subdividing the signal spectrum into partial bands it is possible touse two or more stages. The individual stages can be constituted byfilter bands or transform coding.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a diagrammatic depiction of the subdivision of the spectrumof the input signal into partial bands;

FIG. 1b is a diagrammatic depiction of the combination of the partialband signals to generate an output signal;

FIG. 2 shows a detail of the frequency responses of the partial bandfilters 0 and 1 of a polyphase filter bank;

FIG. 3 shows a detail of the frequency responses of correspondingpartial band filters without using the method according to theinvention;

FIG. 4a is a diagrammatic representation of the weighted summation;

FIG. 4b is a diagrammatic representation of the inverse operation; and

FIG. 5a shows the weighted summation function of FIG. 4a with the filterbank arrangement of FIG. 1;

FIG. 5b shows the inverse weighted summation function of FIG. 4b withthe combining stages of FIG. 1b;

FIG. 6 is a detail of the frequency responses of corresponding partialband filters when applying the method according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Without restricting the general nature of the invention, by way ofexample a description is provided hereinafter of a two-stage method, inwhich the spectrum of the input signal is broken down with the aid of apolyphase filter bank into an even number M of partial bands and using afollowing TDAC (Time Domain Aliasing Cancellation) filter bank, each ofthese partial bands is subdivided into an even number of bands.

The method for subdividing the signal spectrum is shown in FIG. 1a. Apolyphase filter bank 1a breaks down the spectrum of the input signals(n) with the aid of filters into M partial bands (M:even). The pulseresponses of the partial band filter are obtained by multiplication of asample pulse response h_(p) of length L with cosine functions, whosefrequencies correspond to the center frequencies of the partial bands.

The output signals of the filters are underscanned by the factor M (inthe drawing shown as a downward arrow), so that the sum of the scanningrates in all the partial bands remains the same as the scanning rate ofthe input signal.

After filtering and subscanning, the partial bands with uneven indicesare multiplied in multiplier units 3 with the following form {1,-1,1,-1. . . }, so as to cancel out the reflection caused by the filters andwhich leads to spectral components with higher and lower frequenciesoccurring in permutated form in the particular partial bands. For the Mpartial band signals the following applies: ##EQU1##

Each of these M partial bands is subdivided with a following TDAC filterbank 2a into N bands (N:even).

The pulse responses of the TDAC filter bank are obtained bymultiplication with a sample pulse response h_(T) (length 2N) withcorresponding cosine functions. For the M×N partial band signals thefollowing applies: ##EQU2##

The successive filter stages consequently subdivide the input signalinto M×N partial bands.

FIG. 1b shows the combination of M×N partial band signals to form anoutput signal in combination stages 1b and 2b.

In detail form, FIG. 2 shows the frequency responses of two adjacentpartial band filters of a polyphase filter bank with M=32. On thevertical axis is plotted the pulse response in decibels and on thehorizontal axis the frequency standardized to the 384 partial bands ofthe total filter bank (M=32, N=12). The detail extends over thefrequency range of the first 24 partial bands of the total filter bank.The frequency response of the partial band filter (M=0) is indicated bythe continuous line 4 and that of the partial band filter (M=1) with thebroken line 5. It is clearly possible to see the overlap of thefrequency responses of the partial band filters.

As in this transition range there are several partial bands of the totalfilter bank, there is a clear increase of the signal in the frequencyresponse of the corresponding partial band filters outside the passband, i.e., in the stop band. As a result of a vertical exaggeration ofthe frequency responses in the stop band, each spectral component whosefrequency f is in a transition range of the polyphase filter bank,occurs in two partial bands of the total filter bank, namely as a signalcomponent in the partial band with the pass band at frequency f and as acrosstalk component in the partial band with vertical exaggeration atthe frequency f, so that a spectral component of frequency f appears as##EQU3## in the partial band i=N×k+m as a signal component and in thepartial band j=N×k-1-m as a crosstalk component. In addition, the signaland crosstalk components in the corresponding partial bands are at thesame frequency: ##EQU4##

Such an increase in the frequency response in the partial bands of thetotal filter bank is shown in FIG. 3. The latter shows details from thefrequency responses 6, 7 of partial band filter 9 (continuous line) andpartial band filter 14 (broken line), respectively, without using themethod according to the invention for reducing the frequency crosstalk.The input signal, which leads to a signal component in the total partialband 9, leads to a crosstalk component 6A in the total partial band 14.Conversely the input signal, which leads to a signal component in thetotal partial band 14, also leads to a crosstalk component 7A in thepartial band 9.

For the selected example of a polyphase filter bank 1a with followingTDAC filter bank 2a an analysis of the phase relationships gives a phasedifference between the signal and crosstalk components of 180° for-N/2≦m≦-1 and 0° for 0≦m≦N/2-1. if N is an integral multiple of 4. If Nis a non-integral multiple of 2, then the phase difference is 0° for-N/2≦m≦-1 and 180° for 0≦m≦n/2-1.

These phase differences make it possible to reduce the crosstalkcomponents by weighted summation or subtraction of the partial bandsignals x_(i) and x_(j).

FIG. 4a diagrammatically shows the operation of weighted summation withthe weight factors c_(m) and the factors d_(m) derived therefrom; whileFIG. 4b diagrammatically shows the inverse operation with respect to theweighted summation. Due to the uniform structure of the polyphase filterthe number of weight factors to be determined is n/2. The weight factorsare optimized in such a way that by the choice of one optimum weightfactor for each partial band, the crosstalk components are reduced tothe maximum extent.

FIG. 5a shows the weighted summation function of FIG. 4a with the filterbank arrangement of FIG. 1a. As is apparent, the exemplary inputs X_(i)and X_(j) to the weighted summation network 8 in FIG. 5a are therespective outputs X of the filter bank stages 1a and 2a of the filterbank of FIG. 1a. Similarly, FIG. 5b shows the inverse weighted summationfunction 9 of FIG. 4b which outputs representative signals x'_(i) andx'_(j) as inputs to the combining filter stages 1b and 2b of FIG. 1b.

FIG. 6 shows a detail from the frequency responses of the partial bandfilters 9 and 14 with the use of the weighted summation or subtractionprocess according to the invention, and there is a clear improvementcompared with FIG. 3 with regards to the rejection characteristic. Forthis example the optimized weight factors c_(m) are given below:

    ______________________________________                                                m    cm                                                               ______________________________________                                                -6   0.0000                                                                   -5   0.0145                                                                   -4   0.0600                                                                   -3   0.1700                                                                   -2   0.3900                                                                   -1   0.4500                                                           ______________________________________                                    

The optimized weight factors for the combination of the same polyphasefilter bank with a TDAC filter bank, which subdivides the 32 partialbands into in each case a further 18 bands, are:

    ______________________________________                                                m    c.sub.m                                                          ______________________________________                                                -9    0.0000                                                                  -8   -0.0037                                                                  -7   -0.0142                                                                  -6   -0.0410                                                                  -5   -0.0950                                                                  -4   -0.1850                                                                  -3   -0.3300                                                                  -2   -0.5350                                                                  -1   -0.6000                                                          ______________________________________                                    

The output signals of the filter bank modified with the aid of themethod according to the invention correspond to the partial band signalsof a total filter bank with improved frequency responses, because thestop band attenuation of the resulting partial band filters areoptimized by a suitable choice of the weight factors.

Although the invention has been described and illustrated in detail, itis to be clearly understood that the same is by way of illustration andexample, and is not to be taken by way of limitation. The spirit andscope of the present invention are to be limited only by the terms ofthe appended claims.

I claim:
 1. Method for reducing frequency crosstalk in processing ofdigitized input signals, in which input signals are subdivided intopartial frequency bands by means of series connected transformationstages, for subsequent encoding, transmission or storage, followed bydecoding, the signals from the partial bands being again combined toform a signal, said method comprising the steps of:prior to saidencoding, for each partial band performing a weighted summation of apartial band signal thereof, which signal contains spectral componentsof specific frequencies as signal components, and other partial bandsignals from partial bands in which said spectral components of saidspecific frequencies occur as crosstalk components in a stop range;processing the partial band signals by means of an inverse operation tothe weighted summation, following decoding.
 2. Method according to claim1, wherein a first transformation stage of said series connectedtransformation stages subdivides the input signal into an even number Mof partial bands 0 to M-1; and means are provided for causing an outputsignal of uneven numbered partial bands of said stage to undergo acorrection operation before entering a second transformation stage. 3.Method according to claim 2, wherein said correction operation comprisesmultiplying every second time consecutive value of output signals ofsaid uneven numbered partial bands by -1, while the output signals ofthe even numbered partial bands remain unchanged.
 4. Method according toclaim 3, wherein weighted signals y_(i) in the partial band i areobtained in accordance with the following: ##EQU5## in which x_(i) is apartial band signal of partial band i, and c_(m) and d_(m) representweight factors, d_(m) being determined from the weight factors c_(m) bythe above equation.
 5. Method according to claim 2, wherein weightedsignals y_(i) in the partial band i are obtained in accordance with thefollowing: ##EQU6## in which x_(i) is a partial band signal of partialband i, and c_(m) and d_(m) represent weight factors, d_(m) beingdetermined from the weight factors c_(m) by the above equation. 6.Method according to claim 1, wherein weighted signals y_(i) i in thepartial band i are obtained in accordance with the following: ##EQU7##in which x_(i) is a partial band signal of partial band i, c_(m) andd_(m) represent weight factors, d_(m) being determined from the weightfactors c_(m) by the above equation, N is the number of partial bands insaid second transformation stage, k is a partial band index for partialbands of the first transformation stage and m is a partial band indexfor partial bands of the second transformation stage.
 7. Methodaccording to claim 6, wherein said inverse operation is performedaccording to the following equation:

    x.sub.i '=d.sub.m ·(y.sub.i '-c.sub.m ·y.sub.j ')

    x.sub.j '=d.sub.m ·(y.sub.j '-c.sub.m ·y.sub.i ')

in which x_(i) ' is the partial band signal of partial band i afterperforming inverse operation y_(i) ', the weighted partial band signalof partial band i, following encoding and decoding.
 8. Method accordingto claim 6, wherein the weight factors c_(m) for summation are optimizedwith respect to the frequency response of the series-connected stages.9. Method according to claim 8, wherein the number of weight factorsc_(m) to be used for summation is half as large as a number of partialbands produced by a following stage.
 10. Method according to claim 6,wherein the number of weight factors c_(m) to be used for summation ishalf as large as a number of partial bands produced by a followingstage.
 11. Method according to claim 1, wherein said inverse operationis performed according to the following equation:

    x.sub.i '=d.sub.m ·(y.sub.i '-c.sub.m ·y.sub.j ')

    x.sub.j '=d.sub.m ·(y.sub.j '-c.sub.m ·y.sub.i ')

in which x_(i) ' is the partial band signal of partial band i afterperforming the inverse operation, and y_(i) ' is the weighted partialband signal of partial band i, following encoding and decoding. 12.Method according to claim 11, wherein the number of weight factors c_(m)to be used for summation is half as large as a number of partial bandsproduced by a following stage.